1. Field of the Invention
The present invention relates generally to electrical distribution equipment and conductors. The invention relates more particularly to controlling the misdetection of arc faults by arc fault detection systems in electrical systems.
2. Discussion of the Known Art
Switchgear enclosures are commonly employed in electrical power distribution systems for enclosing circuit interrupters and switching equipment associated with the distribution system. Typically, switchgear enclosures are comprised of a number of individual stacked or adjacent compartments, each of the switchgear compartments receiving electrical power from a power source and distributing the electrical power through a feeder circuit to one or more loads. Generally, each of the switchgear compartments includes circuit breakers or other interrupters for breaking electric power in a particular feeder circuit in response to hazardous current overloads in the circuit, or normal switching events.
In addition to current overloads, the switchgear enclosure may encounter other hazardous conditions known as arcing faults. Arcing faults occur when electric current “arcs” or flows through ionized gas between conductors, e.g., between two ends of broken or damaged conductors, or between a conductor and ground in the switchgear enclosure. Arcing faults typically result from corroded, worn or aged wiring or insulation, loose connections and electrical stress caused by repeated overloading, lightning strikes, vermin, dropped wrenches during maintenance, etc. Particularly in medium-to-high voltage power distribution systems, the ionized gas associated with arcing faults may be released at pressures and temperatures sufficient to severely damage or destroy the switchgear equipment and/or cause severe burning injuries or death to operating personnel.
Switchgear enclosures generally provide arc-resistant metal switchgear compartments, often with a means for venting the gases from the compartments in the event of an arcing fault. These compartments are designed to withstand the pressures and temperatures of the gases associated with an arcing fault and reduce the likelihood or extent of damage to switchgear equipment by preventing the gases from entering adjacent switchgear compartments. Safety to operating personnel is enhanced by channeling and venting the hot gases away from operating personnel. However, because these systems do not eliminate the generation and release of hot gases associated with arcing faults, they do not completely eliminate the risk of injury to operating personnel and/or damage to the switchgear equipment.
Both passive and active arc control means are know in the art. Passive means include directed venting of the arc blast energy and gasses out of the cabinet. Other passive means may include reinforcement of the cabinet structure in an effort to withstand the blast. Due to the extremely high power which is generated in an arcing fault, they are typically more damaging, more quickly, than other types of faults and it is therefore desirable to have a system which will respond to these types of faults very quickly to remove the voltage supplying the arc. Limiting arc fault duration through active systems is particularly important in limiting potential damage from the blast. Active means usually include some form of sensing and a switching mechanism to control the current. Of course, the quicker the arc is sensed and controlled the less harm is likely to be done by the arcing event.
One method employed for enhancing the safety and durability of switchgear enclosures in the event of arcing faults, as described in U.S. Pat. No. 5,933,308 to Garzon, is to provide arc-resistant metal switchgear compartments with a fast-acting means for grounding or shunting the source bus current at the feeder breaker for the equipment in the event of an arcing fault condition. This action extinguishes the arc almost immediately by removing the voltage sustaining it, and causes a bolted fault right at the feeder breaker, whether said feeder breaker is the main breaker inside the equipment, or a feeder breaker elsewhere in the electrical system, and the feeder breaker then will break the circuit using its normal protective functions. This is done in Garzon by monitoring the rise rate of the source or main bus current and monitoring the light produced by arcing events in each feeder compartment by optical detectors. The current signal and the optical signal are AND'ed together to produce an arcing fault detection signal which activates an arc diverter mechanism within the appropriate time frame. In another example, as described in U.S. Pat. No. 7,499,251 to Byron, arc fault control is done by detection through a comparison of current on the main bus against the current through the feeder lines, whereby a difference in current gives a first detection signal. The first detection signal can be used directly, or AND'ed with other detection signals from optical detectors on the feeder lines, to provide the activation signal for operating the arc diverter. Other known arcing fault sensing circuits may use only optical detectors. Removal of the voltage sustaining the arc in known systems may be by operation of an arc extinguishing mechanism including the feeder breaker, an arc diverter mechanism, or both, which are responsive to the arc fault detection systems.